This invention relates to a television(TV) with picture-in-picture (PIP) showing two pictures by composing another picture in a portion of a composing picture, particularly to a method and circuit for composing still image to prevent picture shake and reduction of vertical resolution for the PIP.
Generally, a PIP TV has a function for showing a main picture and a sub picture of another image signal in a portion of the main picture. The image signal composing the sub picture in the main picture can be made by reading again the image information in step with a field of the main picture after sampling image data in each field and storing it in memory part, thereby making two pictures.
The conventional TV with such function of PIP includes a typical TV circuit and a PIP circuit. That is, in the conventional TV circuit, a microcomputer(MICOM) controls a tuner and intermediate frequency(IF) amplifier 101 and a signal switching part 103 according to key inputs, then an audio signal amplifier 102 provides an audio signal through a speaker SP. A image-processing and synchronization-separating part 105 provides the image signal to be displayed on a cathode-ray tube(CRT) through a high voltage generation and deflecting part 106 and a CRT driver 113. Next, the PIP circuit reads the image signal for the sub picture in step with the synchronization signal of the main picture after storing the image signal for the sub picture in the memory part, in order to compose the PIP.
In more detail, the image signal for the sub picture selected by the signal switching part 103 is divided into a synchronization signal, a luminance signal and a color signal at the synchronization-separating and image signal-processing part 104. The synchronization signal is provided to a clock generator 107, while the luminance and color signals are provided to a analog-to-digital(A/D) converter 108.
Next, after an data control part 109 stores the analog-to-digital converted data in accordance to the generated synchronization signal and clock signal from the clock generator 107 in the memory part 110, the stored data of the memory part 110 are read with the clock signal of the clock generator 107 in step with the synchronization signal for the main picture and provided to the digital-to-analog(D/A) converter 111.
The D/A converter 111 converts the stored image data back into the original analog signal as provided to the A/D converter 108. The slowly sampled data during the A/D conversion are stored in the memory part, but the stored data are rapidly read during the D/A conversion. Thus, the analog signal output of the D/A converter is time-compressed. This compressed image signal is provided to the signal switching combination part 112, so the compressed image signal is allocated in a portion of the main picture on the CRT.
In order to carry out such a function of PIP, the main image signal is displayed on the main picture area of the CRT in step with the synchronization signal, while the image signal for the sub picture is displayed on a sub picture area of the CRT in step with the scanning field. That is, the image signal for the main picture forms one picture (refer to FIG. 2(C)) by using the scanning method consisting of the first field (refer to FIG. 2(A)) and second field (refer to FIG. 2(B)), while the image signal for the sub picture is scanned in synchronization with the scanning method of the main picture during reading from the memory part.
But, the image signal field of the sub picture occasionally becomes reverse to that of the main picture as shown in FIG. 3. In such case, a plurality of field memories (usually 4 memories) are used to prevent the field reversal between the main picture and sub picture. In other words, the first and second image signal field of the sub picture are stored into 2 respective memories, that is, a total of 4 memories. In reading, the unused memory is read and then the first field and the second field are displayed according to the field consistent with the main picture.
But in this PIP, the resolution is degraded when making a still picture. The reason is that horizontal compression is possible since the data can be rapidly read from the memories after slowly storing the data, while vertical compression requires the reduction of a number of lines on characteristics of the sub picture being inserted into a portion of the horizontal line for the main picture.
Such phenomena can be easily understood from the conventional TV signal composition and the PIP signal composition diagram shown in FIG. 4(A) and (B). The first field of the main picture provides first line data of a first field for the sub picture, and omits second and third line data, and provides fourth line data, thereby forming the sequence of 1, 4, 7, 10 . . . (refer to FIG. 4(B)). Next, the second field of the main picture provides inter-line data of the first line data, that is, it forms the sequence of 13, 16, 19 . . . (refer to FIG. 4 (B)). Thus, the PIP is formed by the first and second field as mentioned above. At this time, a still picture is made up by reading repeatedly the stored sub picture data in the memory. When the still picture is selected, the sub picture of the PIP consists of the first field and the second field of the still picture as shown in FIG. 4(B) so that the image change between the first and second field pictures is repeated, thereby producing picture shake.
That is, there is picture shake due to the time interval between the first field and the second field. To prevent such a shake phenomenon, the conventional PIP is made up by composing the sub picture as shown in FIG. 4(C).
The still picture is made up by providing continuously the data corresponding to one field of the stored sub picture image signal in the memory according to both the first field and the second field without distinguishing the fields from each other. But, in this case the amount of data provided for the real picture reduces to a half since the data corresponding to only one field of the first field and the second field are read, so that the vertical resolution is reduced by a half.